Sage Sankara:~ Without knowing and examining the external world, one can’t know the Truth.+

Sage Sankara says: When the whole universe, movable and immovable, is known to be Atman, and thus the existence of everything else is negated, where is then any room to say that the body is Atman.

If the body is not the Self, then the karma theory has no meaning from the standpoint of the Soul, the Self, which is bodiless because it is formless, timeless, and spaceless existence. 

Thus, no second thing exists other than the Soul, which is present in the form of the consciousness.

That is why Sage Sankara says: ~ What is accepted without a proper inquiry will not lead to the final Goal. (Commentary on Vedanta Sutra).

Sage Sankara:~ VC.63- Without knowing and examining the external world, one can’t know the Truth, as the idea that the external world exists, won't go. It can go only by an inquiry into the nature of the external world.

If the body is not the Self, then the experience of birth, life, death, and the universe are bound to be an illusion. 

If the experience of birth, life, death, and the universe is merely an illusion then all conduct, actions, and their fruits are mere illusion. : ~ Santthosh Kumaar